Equipment Condition Considerations
Keeping the objective in mind, a general purpose (GP) grease is used as a multi-application grease in a production process to reduce complexity and the potential for component failure due to misapplication. Consider the following operational characteristics when selecting the GP grease.
Size and Type
Ball screws, cables, linear bearings, plain bearings, rolling element bearings, slide-ways and seals are just a few of the many different component types that are grease lubricated. If sliding friction is the dominant contact type, then there may be a greater reliance on heavy viscosity oils, polymers and solid additives to support the load and provide lubricating film protection.
If rolling friction is the dominant contact type, then greases with lighter viscosity base oils and minimal use of polymers, solids and antiwear (AW) and extreme pressure (EP) additives can be effectively used.
Load
As the load increases, the grease’s base oil viscosity must also increase to support the load. If the majority of the components in the mill/plant environment are heavily loaded, it may make sense to use high-viscosity base oils for a general purpose product. This might be the case in a cement, steel or paper mill environment. It is not uncommon to find GP greases made from 460 cSt (40°C) and heavier oils in these types of environments.
Speed
As the speed increases or the load diminishes, the required base oil viscosity also diminishes. In operations with predominantly moderate to high-speed and lightly to moderately loaded applications, the grease’s oil viscosity would fall to an ISO 46 to 150 range. It is unusual to find highly loaded applications that also operate at high speeds that are lubricated with grease. This type of application would likely warrant special consideration and therefore fall outside this discussion.
Atmosphere
The three atmospheric factors that must be accounted for are temperature, moisture and airborne solid contaminants (particles). Although the influence of atmospheric factors can be significant, these factors are considered after the viscosity selection is complete.
Lubrication Intervals
The method of application combined with the application cycle dictates the rate of application. The rate of relubrication is the amount of lubricant fed into the component in a given time.
Greased components require a constant supply of lubricant at the load zone to sustain the hydrodynamic film much the same as oil lubricated components. The reserve grease contained in the cavity in the housing serves as an oil reservoir that components draw from for lubrication.
When grease is resupplied to the housing, the oil reservoir is replenished. The longer the duration between cycles, the greater the likelihood that the reservoir will deplete and the component will run to a semi-dry (mixed film) condition.
The oil in the load zone is squeezed and pushed away over time. If the relubrication volume is insufficient, or the cycle is sporadic (greater risk with manual lubrication), the likelihood that the oil film will dissipate leading to mixed film conditions increases. When these conditions are prevalent, the grease selection must be one that resists the squeezing action and tendency to dissipate. Greases formulated with heavier viscosity base oils and chemical and mechanical film forming additives can be helpful in these circumstances.